1. Field of the Invention
The present invention relates to a separation method for separating solid substances from each other, for example, a separation method for separating fine powder attached on a grain from the grain and, more specifically, a method of separating lightweight grains (flosses) from raw material including plastic pellets and flosses and the like or a method of separating bran from grain, and to a device for performing these methods.
2. Description of the Related Art
Plastic material for injection molding is provided in the state of pellets in many cases. While storing or transporting this material, filamentous or powdery, so-called, flosses or lightweight grains are generated by being broken or ground down by friction and thus are mixed with raw grains. When the material including such lightweight grains is stirred and heated, these grains melt out from the pellets. However, since flosses can hardly be melted, fine grains remain in the melted substance as foreign bodies.
For example, when molding a resin lens, the percentage of conforming articles is in the order of 80% due to the existence of such remaining foreign bodies. There may be a case in which the percentage of conforming articles is lower than 20% when the material contains the flosses even a little. Therefore, complete removal of the flosses is required, but it is difficult to remove the flosses completely with the device in the related art.
A device called “floss separator” for removing the flosses is known. FIG. 21 is a schematic drawing of this device. Resin material containing the flosses fed through a pneumatic transport pipe is injected by a feeding unit 17 into a cylindrical section 1 toward the inner wall of the pipe in the direction in which the pellets and the flosses rotationally move upward at a high-speed. An exhaust blower, not shown, is connected to an exhaust pipe 2 at the upper portion of the cylindrical section 1, and hence air and the flosses in the cylindrical section 1 are taken out via the exhaust pipe 2. On the other hand, the pellets move upward in whirling motion while rolling on the wall surface, and are separated from the flosses during this process. Consequently, the pellets move downward by gravity, and are taken out from the lower end of a conical section 3.
In general, when fine powder is contained in the grains, such fine powder may be separated by using a sieve. However, in the case where flosses are attached to the plastic pellets with electrostatic action, the flosses cannot be separated in the separation method in the related art described above.
In order to increase the rate of floss removal, an attempt has been made to increase the length H of the cylinder, or more specifically, the length of the portion of the device above the feeding unit 17. However, even with such configuration, it is still difficult to remove the flosses by 100%. Therefore, development of the method which can remove the flosses by 100% has been strongly required by those in charge of injection molding.
In the case where the flosses are attached to the plastic pellets by electrostatic action described above, that is, when fine grains are attached to the grains, there arises a problem in that when blowing the lightweight powder bodies upward by airflow to separate the powder bodies and grains, the grains are also blown upward. In particular, when the grains are light, they cannot be separated easily.